This invention relates to the treatment of metal surfaces prior to the application of a siccative organic coating. Specifically, this invention relates to the treatment of conversion-coated metal with an aqueous solution comprised of 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethylsilane and one or more Group IVA metal ions.
The primary purposes of applying siccative coatings (also known as xe2x80x9corganic coatingsxe2x80x9d, xe2x80x9corganic finishesxe2x80x9d, or simply, xe2x80x9cpaintsxe2x80x9d) to metal substrates such as steel, aluminum, zinc and their alloys, are protection of the metal surface from corrosion and for aesthetic reasons. It is well known, however, that many organic coatings adhere poorly to metals in their normal state. As a result, corrosion-resistance characteristics of the siccative coating are substantially diminished. It is therefore a typical procedure in the metal finishing industry to subject metals to a pretreatment process whereby a conversion coating is formed on the metal surface.
This conversion coating acts as a protective layer, slowing the onset of the degradation of the base metal, owing to the conversion coating being less soluble in a corrosive environment than is the base metal. The conversion coating also serves as a recipient for the subsequent siccative coating. The conversion coating has a greater surface area than does the base metal and thus provides for a greater number of adhesion sites for the interaction between the conversion coating and the organic finish. Typical examples of such conversion coatings include, but are not limited to, iron phosphate coatings, zinc phosphate coatings, and chromate conversion coatings.
Normally, the application of an organic finish to a conversion-coated metal surface is not sufficient to provide the highest levels of paint adhesion and corrosion resistance. Painted metal surfaces are able to reach maximum performance levels when the conversion-coated metal surface is treated with a xe2x80x9cfinal rinsexe2x80x9d, also referred to in the art as a xe2x80x9cpost-rinsexe2x80x9d or a xe2x80x9cseal rinsexe2x80x9d prior to the painting operation. Final rinses are typically aqueous solutions containing organic or inorganic entities designed to improve paint adhesion and corrosion resistance. This may be accomplished by altering the electrochemical state of the conversion-coated substrate by rendering it more passive or it may be accomplished by forming a barrier film which prevents a corrosive medium from reaching the metal surface.
The most effective final rinses in general use today are aqueous solutions containing chromic acid, partially reduced to render a solution comprised of a combination of hexavalent and trivalent chromium. Final rinses of this type have long been known to provide the highest levels of paint adhesion and corrosion resistance. Chromium-containing final rinses, however, have serious drawbacks due to their inherent toxicity and hazardous nature. These concerns make chromium-containing final rinses less desirable from a practical standpoint, when one considers such issues as safe handling of chemicals and the environmental problems associated with the discharge of such solutions into municipal water streams. Thus, it has been a goal of the industry to find chromium-free alternatives which are less toxic and more environmentally benign than chromium-containing final rinses while also providing comparable effectiveness with regard to paint adhesion and corrosion resistance properties.
A final rinse composition containing 3-aminopropyltriethoxysilane and a titanium chelate is disclosed in U.S. Pat. No. 5,053,081.
A final rinse composition containing an organosilane selected from methyltrimethoxysilane and phenyltrimethoxysilane and mixtures thereof and Group IVA metal ions is disclosed in U.S. Pat. No. 5,531,820.
A final rinse composition containing an organosilane selected from 3-glycidoxypropyltrimethoxysilane, methyltrimethoxysilane, xcex3-methacryloxytrimethoxysilane and phenyltrimethoxysilane, and mixtures thereof and zirconium ion is disclosed in Re. 35,688.
I have discovered that aqueous solutions containing 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethylsilane and Group IVA metal ions, namely, zirconium, titanium or hafnium, or mixtures thereof, provide paint adhesion and corrosion resistance characteristics comparable or superior to those attained with chromium-containing final rinses in accelerated corrosion tests.
Accordingly, in its principal aspect, this invention is an aqueous composition comprising
i) about 0.06 to about 25 weight percent 3-aminopropyltriethoxysilane;
ii) about 0.06 to about 25 weight percent 3-glycidoxypropyltrimethylsilane;
iv) up to about 5 weight percent of one or more alcohols; and
v) about 0.005 to about 25 weight percent of a Group IVA metal ion selected from zirconium, hafnium and titanium, or a mixture thereof, wherein said aqueous solution has a pH of about 2 to about 9.
The aqueous composition of this invention is a suitable replacement for final rinse products that contain chromates as it is safe and does not require expensive waste treatment procedures, thereby eliminating the hazards and expense associated with chromate-containing products.
The aqueous solution of this invention is prepared by dissolving the 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethylsilane in one or more water miscible alcohol solvents and then adding an aqueous solution containing zirconium, titanium or hafnium ions, or mixtures thereof, with mixing to the silanes such that the pH of the resulting solution is in the range of about 2.0 to about 9.0. Preferred water miscible alcohol solvents include methanol, ethanol, isopropanol, and the like. Ethanol is more preferred. The solution is then mixed for at least 30 minutes to complete the hydrolysis of the organosilanes.
The pH of the aqueous solution is then measured. If the pH is outside the desired range, water or Group IVA metal salt is added to bring the pH within the desired range. Hence, the amount of Group IVA metal ion present in the finished solution is a function of the pH. The aqueous solution of this invention typically contains Group IVA metal ions at a concentration of at least about 0.005 weight percent.
The source of the zirconium, hafnium or titanium ions can be hexafluorozirconic acid, zirconium basic sulfate, zirconium hydroxychloride, zirconium basic carbonate, zirconium oxychloride, zirconium acetate, zirconium fluoride, zirconium hydroxide, zirconium orthosulfate, zirconium oxide, zirconium potassium carbonate, hexafluorotitanic acid, hafnium oxychloride, and the like, and mixtures thereof. When zirconium-containing salts such as zirconium basic sulfate, zirconium hydroxychloride, zirconium basic carbonate, zirconium oxychloride are used as the zirconium source, the salts must be dissolved in 50% hydrofluoric acid in order to effect dissolution.
The aqueous composition may be prepared on-site for immediate use as described herein, or prepared as a concentrate for shipping to the site for dilution to the desired concentration with water.
In a preferred aspect, the aqueous solution is prepared as a concentrate comprising about 12.5 to about 25 weight percent 3-aminopropyltriethoxysilane, about 12.5 to about 25 weight percent 3-glycidoxypropyl and about 11.8 to about 25 weight percent of the Group IVA metal ion.
In a preferred aspect of this invention, the aqueous composition of has a pH is about 4 to about 5.
In another preferred aspect, the Group IVA metal ion is zirconium.
The formation of conversion coatings on metal substrates is well known within the metal finishing industry. Metal substrates suitable for treating are primarily steel, but any metal surface that can accept a conversion coating is acceptable. These include, but are not limited to, iron, steel, aluminum, aluminum alloys, zinc, zinc alloys and other non-ferrous metals that can be conversion coated. Cold-rolled steel, hot-rolled steel, aluminum and zinc-coated (galvanized) metals are especially suitable.
In general, the conversion coating process is usually described as a process requiring several pretreatment stages. The actual number of stages is typically dependent on the final use of the painted metal article. The number of pretreatment steps normally varies anywhere from two to nine stages.
A representative example of a pretreatment process involves a five-stage operation where the metal to be ultimately painted goes through a cleaning stage, a water rinse, a conversion coating stage, a water rinse and a final rinse stage. The final rinse (also referred to as a seal rinse) provides an additional level of paint adhesion and corrosion resistance to the conversion-coated metal surface. The intervening water rinses are needed to wash away any excess chemical from the preceding step to prevent drag-in into subsequent steps.
Modifications to the pretreatment process can be made according to specific needs. For example, surfactants can be incorporated into some conversion coating baths so that cleaning and the formation of the conversion coating can be achieved simultaneously. In other cases it may be necessary to increase the number of pretreatment stages so as to accommodate more pretreatment steps.
Examples of the types of conversion coatings that can be formed on metal substrates are iron phosphates and zinc phosphates. Iron phosphating is usually accomplished in no more than five pretreatment stages, while zinc phosphating usually requires a minimum of six pretreatment stages. The number of rinse stages between the actual pretreatment steps can be adjusted to insure that rinsing is complete and effective so that the chemical pretreatment from one stage is not carried on the metal surface to subsequent stages, thereby possibly contaminating them. It is typical to increase the number of rinse stages when the metal parts to be treated have unusual geometries or areas that are difficult for the rinse water to contact.
Accordingly, in another aspect, this invention is a method of treating a conversion coated metal surface comprising applying to the metal surface an aqueous composition comprising i) about 0.06 to about 7 weight percent 3-aminopropyltriethoxysilane; ii) about 0.06 to about 7 weight percent 3-glycidoxypropyltrimethylsilane; iii) up to about 5 weight percent alcohol; and iv) about 0.005 to about 0.25 weight percent of a Group IVA metal ion selected from zirconium, hafnium and titanium, or a mixture thereof, wherein said aqueous solution has a pH of about 2 to about 9.
In another preferred aspect, an aqueous composition comprising i) about 0.25 to about 2 weight percent 3-aminopropyltriethoxysilane; ii) about 0.25 to about 2 weight percent 3-glycidoxypropyltrimethylsilane and iii) about 0.005 to about 0.25 weight percent of zirconium ion, wherein said aqueous solution has a pH of about 4 to about 5 is applied to the metal surface.
The aqueous composition can be applied to the conversion coated metal by any number of conventional means including immersion, spraying, brushing, rolling, and the like. Preferably, the aqueous composition is applied using either an immersion or a spray operation.
In immersion operations, the metal articles are submersed in the various pretreatment baths for defined intervals before moving on to the next pretreatment stage. A spray operation is one where the pretreatment solutions and rinses are circulated by means of a pump through risers fashioned with spray nozzles. The metal articles to be treated normally proceed through the pretreatment operation by means of a continuous conveyor. Virtually all pretreatment processes can be modified to run in spray mode or immersion mode, and the choice is usually made based on the final requirements of the painted metal article. It is to be understood that the invention described herein can be applied to any conversion-coated metal surface.
The aqueous composition of this invention is applied at temperatures about 40xc2x0 F. to about 180xc2x0 F., preferably about 60xc2x0 F. to about 90xc2x0 F.
The treated conversion-coated metal article is then dried by various means including air drying, forced air drying and oven drying.
The metal article is now ready for application of the siccative coating. The treated surface is, however, capable of withstanding relatively mild or short term corrosive conditions without further finishing or coating.
Accordingly, in another aspect, this invention is a treated conversion coated metal surface prepared by applying to a conversion coated metal surface an aqueous composition comprising
i) about 0.06 to about 7 weight percent 3-aminopropyltriethoxysilane;
ii) about 0.06 to about 7 weight percent 3-glycidoxypropyltrimethylsilane;
iii) up to about 5 weight percent of one or more alcohols; and
iv) about 0.005 to about 0.25 weight percent of a Group IVA metal ion selected from zirconium, hafnium and titanium, or a mixture thereof, wherein said aqueous solution has a pH of about 2 to about 9.
The foregoing may be better understood by reference to the following Examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention.